Firefox not in Ubuntu Software Repository

Firefox is not appearing in the Ubuntu Software repository to install. I don't understand why, because it is the default browser when you install Ubuntu.
I installed the latest version from Mozilla's website with their installation instruction, because the current one was outdated. This caused some integration issues, because a link within a document did not want to open in Firefox when I clicked on it.
I thought I could fix it by uninstalling the latest version and current version, and re-install the standard one from the Ubuntu Software repository, only to find that it is not available there.
Can anyone tell me why it is not available in the Ubuntu Software repository?

The Firefox version installed was 53 or 54 something, and the Software Updater did not have the latest one yet. I had trouble, still have, with Flash Player videos that struggle to stream. On my work's Windows laptop, they stream fine, but not on my Linux Firefox. I was hoping maybe the latest Firefox has something built-in or better Flashplayer support.

If I search for Firefox on my Ubuntu Software, the only results I get is:
Search Firefox Bookmarks Provider
Iceweasel